The invention concerns an interface layer for incorporation in devices produced on a transparent, electrically conducting electrode.
A number of electrical devices are known which are concerned with the generation or modification of light and which comprise an active region located between two essentially planar electrodes. In order to facilitate light transmission from (or to) the device, at least one electrode must be transparent.
Such devices include organic light emitting diodes (OLED) and electrochromic devices.
In OLEDs, an emissive electroluminescent layer comprising one or more layers of organic compound is located between the electrodes and emits light in response to an applied voltage.
In electrochromic devices a stack of materials which, in combination exhibit electrochromic properties is located between the electrodes and changes colour and, or opacity in response to an applied voltage.
Devices of the type which concern the invention are typically fabricated by providing a transparent conducting electrode comprising a transparent substrate and a conductive coating stack, and building successive layers thereon comprising the active region of the device and a further electrode—which may also be transparent. The transparent conducting electrode is frequently realised by depositing the conductive stack of coatings on the substrate using techniques such as chemical vapour deposition (CVD), which are well known to persons skilled in the art, see for example U.S. Pat. No. 7,968,201.
The conductive stack typically comprises a Transparent Conducting Oxide (TCO), i.e. a doped metal oxide, as the uppermost layer (i.e. the furthest layer from the substrate). In addition to offering the requisite electrical properties and mechanical stability, the TCO should offer a suitable surface for deposition of further layers as the rest of the device is fabricated. Examples of transparent conductive oxide materials include fluorine doped tin oxide (SnO2:F), Zinc oxide doped with Aluminium, Gallium or Boron (ZnO:Al, ZnO:Ga, ZnO:B), Indium oxide doped with tin (ITO) and cadmium stannate.
Unfortunately, these surfaces can be inherently rough which can cause localised short circuits drawing current from an area up to a few millimeters from the point of short circuit. This results in a undesirable aesthetic effect as well as reducing the performance of the device.
Furthermore defects such as pinholes in the devices deposited on the TCO can result in similar problems.
The current invention addresses both of these problems.
According to the invention a transparent electrode comprises the features set out in claim 1 attached hereto.
The inventors have shown that incorporation of a thin interface layer between the top of the electrode stack (TCO) and the active region of the device overcomes problems associated with roughness of the TCO. Examples of interface layer materials include TiO2, SiO2, SnO2 and ZnO and mixtures comprising any of these. The good device performance obtained when such interface layers are included is surprising because these materials have high electrical resistance.
In a preferred embodiment, the interface layer comprises a TiO2 layer having a thickness greater than 5 nm.
In another preferred embodiment, the interface layer comprises ZnO, having a thickness between 25 and 80 nm.
In a further preferred embodiment, the TCO layer comprises a fluorine doped tin oxide.
In some embodiments, the underlayers comprise a layer of SnO2 and a layer of SiO2.
The transparent conducting electrode according to the invention is suitable for incorporation in electronic devices such as electrochromic devices and organic light emitting diodes.
According to a second aspect of the invention, a method of manufacturing a transparent conducting electrode comprises the steps set out in claim 8 attached hereto.
In a preferred embodiment, the underlayers, TCO and interface layer are deposited by Chemical Vapour Deposition (CVD). The CVD may be done on the float glass ribbon produced during the float glass production process.
In another preferred embodiment, the underlayers, TCO and interface layer are deposited by Plasma Enhanced CVD.
In another preferred embodiment, the underlayers, TCO and interface layer are deposited by sputtering.
Preferably, the interface layer is selected from TiO2, SiO2 and ZnO. More preferably, the interface comprises TiO2 having a thickness of less than 5 nm.
In another preferred embodiment, the interface layer comprises ZnO, having a thickness between 25 and 80 nm.
In some embodiments, the TCO layer comprises a fluorine doped tin oxide.
In some embodiments, the underlayers comprise a layer of SnO2 and a layer of SiO2.
Materials such as TiO2 and ZnO are is rendered more hydrophilic upon exposure to ultraviolet (UV) radiation. Such treatment of interface layers according to the invention renders the electrode more receptive to deposition of subsequent layers as the device is fabricated, particularly where such layers are deposited by so-called ‘wet’ chemical methods i.e. techniques involving liquid solutions.
In another aspect of the invention, use, as a transparent conducting electrode, of an electrically conducting coating stack located on a transparent substrate;
the stack comprising at least one underlayer and a transparent conducting oxide (TCO) layer located on the underlayers,
is characterised by the electrode having an interface layer located on the transparent conducting oxide layer.
All of the figures are for illustrative purposes only and the relative thicknesses of the layers are not to scale.